Since being excellent in heat resistance, chemical resistance, corrosion resistance, electrical properties, and the like, and having a high elastic modulus, an epoxy resin has been widely used in various applications, such as a sealing material for electric and electronic materials, a matrix resin represented, for example, by a fiber reinforced composite material for sport application, aircraft materials, and the like, a laminate for electrical use including a printed circuit board, an automobile component, a coating material for civil engineering and construction, and a structure adhesive.
However, disadvantageously, since the epoxy resin is inferior in toughness, is fragile, and is liable to be chipped, and in addition, the adhesion of the epoxy resin to various substrates is also not sufficient, these properties thereof are required to be improved.
Heretofore, in order to improve the toughness of a thermosetting resin composition mainly composed of an epoxy resin so as not to be easily fractured, addition of a reactive rubber or polymer particles having a core/shell structure has been generally performed. As the reactive rubber for this purpose, for example, a carboxyl-terminated random copolymer of butadiene and acrylonitrile (CTBN) and an amino-terminated random copolymer of butadiene and acrylonitrile (ATBN) have been known, and as the polymer particles having a core/shell structure, for example, polymer particles containing a poly(butyl acrylate) or a polybutadiene as the core and a poly(methyl methacrylate) as the shell has been known (see Non-Patent Documents 1 and 2).
However, in the case of the conventional techniques described above, in order to obtain a sufficient effect of improving a fracture toughness, a large amount of the reactive rubber or the polymer particles having a core/shell structure must be added to an epoxy resin, and as a result, there has been a problem in that excellent heat resistance and elastic modulus inherent in the epoxy resin are degraded.
In addition, in order to obtain a prepreg excellent in heat resistance and impact resistance, an epoxy resin composition for a prepreg has been proposed in which an epoxy resin curing agent and a thermoplastic resin, such as a polycarbonate, a polysulfone, a poly(ether sulfone), a poly(ether imide), or an aromatic polyester, are blended with an epoxy resin (see Patent Document 1).
However, this epoxy resin composition has a high viscosity and is inferior in handling properties and processability, a cured resin obtained from this epoxy resin composition is not satisfactory in terms of heat resistance and chemical resistance, and furthermore, incompatible portions and/or voids may be generated in the cured resin in some cases.
Furthermore, a technique has been proposed in which impact resistance and toughness of an epoxy resin are improved by blending a block copolymer therewith, and as the block copolymer in this case, there are mentioned a block copolymer of polystyrene-polybutadiene-poly(methyl methacrylate), a block copolymer of polybutadiene-poly(methyl methacrylate), a block copolymer of poly(methyl methacrylate)-polybutadiene-poly(methyl methacrylate), or a block copolymer of poly(methyl methacrylate)-poly(butyl acrylate)-poly(methyl methacrylate) (see Patent Documents 2 to 4).
However, in the above conventional techniques in which a block copolymer is blended with an epoxy resin, when a block copolymer containing a polybutadiene block (a block copolymer of polystyrene-polybutadiene-poly(methyl methacrylate), a block copolymer of polybutadiene-poly(methyl methacrylate), or a block copolymer of poly(methyl methacrylate)-polybutadiene-poly(methyl methacrylate)) is blended, the toughness and the impact resistance of the epoxy resin are improved to a certain extent; however, due to unsaturated double bonds of the polybutadiene block, the weather resistance of a cured resin obtained from the epoxy resin composition is liable to be degraded.
In addition, when the inventors of the present invention investigated properties of a cured resin which was obtained by curing a curable resin composition prepared by blending the block copolymer of poly(methyl methacrylate)-poly(butyl acrylate)-poly(methyl methacrylate) disclosed in Patent Documents 2 to 4 with an epoxy resin together with an epoxy resin curing agent, it was found that although the cured epoxy resin obtained from the epoxy resin composition containing the above block copolymer had an excellent weather resistance since containing no unsaturated double bonds, compared to the case in which the block copolymer of polystyrene-polybutadiene-poly(methyl methacrylate) was used, the effect of improving a fracture toughness was low. Furthermore, it was also found that compared to the case in which the polymer particles having a core/shell structure disclosed in Non-Patent Documents 1 and 2 was used, the effect of improving a fracture toughness was equal to or less than that thereof, and a sufficiently satisfactory fracture toughness was not obtained.
In particular, according to the epoxy resin composition disclosed in Patent Document 4, as the epoxy resin curing agent, an aromatic polyamine, such as diaminodiphenylsulfone, is exclusively used; however, the fracture toughness of a cured resin cannot be considered to be sufficient which is obtained by curing an epoxy resin composition prepared by blending the block copolymer of poly(methyl methacrylate)-poly(butyl acrylate)-poly(methyl methacrylate) actually used in one example of Patent Document 4 with an epoxy resin together with a diaminodiphenylsulfone curing agent.
In addition, in general, when the fracture toughness of a cured resin or the like is improved, concomitant therewith, the peel adhesive strength is improved, and on the other hand, the shear adhesive strength tends to decrease; hence, a cured resin has been desired which improves the shear adhesive strength as well as improving the fracture toughness and the peel adhesive strength.    [Patent Document 1]: Japanese Examined Patent Application Publication No. 6-43508    [Patent Document 2]: Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2003-535181    [Patent Document 3]: WO2007/009957    [Patent Document 4]: Japanese Unexamined Patent Application Publication No. 2007-154160    [Patent Document 5]: Japanese Unexamined Patent Application Publication No. 6-93060    [Patent Document 6]: Japanese Examined Patent Application Publication No. 7-25859    [Patent Document 7]: Japanese Unexamined Patent Application Publication No. 11-335432    [Non-Patent Document 1]: P. Lovell, “Macromol. Symp.”, 92, 1995, p 71-81    [Non-Patent Document 2]: A. Maazouz et al. “Polymer Material Science Engineering”, 70, 1994, p 13-14    [Non-Patent Document 3]: G. Moineau et al. “Macromol. Chem. Phys.”, 201, 2000, p. 1108-1114    [Non-Patent Document 4]: A. Bonnet et al. “International SAMPLE Symposium and Exhibition”, 50, 2005, p 847-854    [Non-Patent Document 5]:r. H. Hydro et al. “J. Polym. Sci., Part B: Polym. Phys.”, 45(12), 2007, p. 1470-1481